With increasing communication traffic, a large number of transmission networks have been configured on the basis of wavelength division multiplexing technology in a long hole area, a metro area, and an access area by a user. To configure a flexible optical transmission network, an optical add-drop multiplexer (OADM) capable of passing through, adding or dropping a node in an optical wavelength unit is demanded.
In an optical add-drop multiplexer, a terminal station device, a relay device, etc., a level adjustment is performed for each wavelength by monitoring a wavelength division multiplexed signal to realize a long distance transmission.
An optical add-drop multiplexer has a wavelength selection switch (WSS) for adjusting the level of an optical signal of each wavelength. The wavelength selection switch demultiplexes wavelength division multiplexed light into each wavelength by a demultiplexer, and selects passage of a thru signal or an add signal by an optical switch. After the attenuation by a variable optical attenuator (VOA), a multiplexer performs a wavelength division multiplexing operation. The level of the wavelength division multiplexed signal is monitored by an optical channel monitor (OCM), and the amount of attenuation of the variable optical attenuator is controlled so that the signal of each wavelength can reach a target level.
As a wavelength selection switch, a micro electro mechanical system (MEMS) has become dominant because of its advantage in signal transmission band characteristic, loss, and polarization dependence.
To reduce the entire cost in the optical add-drop multiplexer, the monitoring process on the input side of the device is omitted, the optical signal level of each wavelength is monitored in the subsequent stage of the wavelength selection switch, and the amount of attenuation is controlled.
FIG. 15 illustrates a configuration of a wavelength selection switch 11 using a MEMS. The wavelength selection switch 11 includes a collimator 12, a grating 13, a lens 14, and an MEMS mirror 15.
The light input from each of input ports 2 through 4 is converted by a collimator 12 into spatial light, and demultiplexed by the grating 13 into light of each wavelength. The demultiplexed light is condensed by the lens 14, and amount of attenuation of the light of each wavelength is controlled by the MEMS mirror 15 and output to an output port 1. The MEMS mirror 15 has a mirror for each demultiplexed wavelength, and each mirror is arranged orthogonal (direction x in FIG. 15) to the array direction (direction y in FIG. 15) of the ports 1 through 4.
FIG. 16 illustrates a controlling operation of the wavelength selection switch 11 of the related art. FIG. 16 illustrates an optical coupler (CPL) 21, an optical channel monitor (OCM) 22, and a control unit 23.
The optical coupler (CPL) 21 branches the output light of the output port 1 and outputs the light to the optical channel monitor 22. The optical channel monitor 22 monitors the optical signal level of each wavelength. The control unit 23 controls the angle of the MEMS mirror 15 depending on the detection level of the optical channel monitor 22.
For example, the optical signal of the ch 16 of the input port 4 is output to the port 1, the control unit 23 controls the angle of the mirror corresponding to a channel ch 16 of the MEMS mirror 15 so that the signal level of the output port 1 can be a desired value. The mirror is provided for each channel, and looks overlapping actually in the position of the ch 16 with reference to FIG. 15.
FIGS. 17A and 17B are explanatory views of the control direction of the MEMS mirror 15. FIG. 17A is an example of the case in which the angle of the MEMS mirror 15 is controlled in the array direction (direction y) of the port. FIG. 17B is an example of the case in which the angle of the MEMS mirror 15 is controlled in the direction (direction x) orthogonal to the array direction of the port for attenuation.
The optical path a indicated by broken lines in FIG. 17A is an optical path when the angle of the MEMS mirror 15 is changed in the direction y. By adjusting the angle of the MEMS mirror 15, the amount of the light input to the output port 1 can be increased or decreased to control the amount of attenuation.
Since there is the possibility in this control method that light leaks (cross talk) to an adjacent port when the amount of shift of the optical axis is increased to increase the amount of attenuation, it is necessary to increase the distance between ports to be equal to or larger than a predetermined value, thereby causing the problem that the entire device becomes large. In addition, when the angle of the mirror is controlled in the array direction of ports, the angle control tolerance (allowance of the angle control) of the MEMS mirror 15 becomes low, it is necessary to control the amount of attenuation of the MEMS mirror 15, that is, the angle of the mirror, with high accuracy.
FIG. 17B is a top view from the arrow direction in FIG. 17A. When the angle of the MEMS mirror 15 is controlled so that it can be orthogonal to the array direction of the ports 1 through 4, the light does not leak to the adjacent port although the angle is expanded. Therefore, there occurs no problem of cross talk, but there occurs another problem that the transmission band characteristic of a signal is degraded.
The degradation of a transmission band characteristic is described in the patent document 3 as follows. That is, since the amount of reducing the reflected light beam from the vicinity of the end surface of a mirror (movable reflecting object) increases, the influence of diffraction becomes outstanding, thereby indicating an inverted trapezoidal amount of attenuation to band characteristic. As a result, the amount of attenuation to band characteristic of the reflected light of a mirror does not indicate a trapezoidal shape, but indicates a substantially M shape referring to an increased amount of attenuation in a certain wavelength band, thus degrading the transmission band characteristic.
Described below concretely is the control method of the MEMS mirror 15. FIG. 18 is a boot-up sequence. FIG. 18 is an example of the casein which the angle of the MEMS mirror 15 is controlled in the array direction of the port during the boot-up.
When the optical input level is equal to or lower than the threshold (S11 in FIG. 18), the angle of the MEMS mirror 15 is adjusted in the direction of the ports so that the fixed amount of attenuation can be assigned (S12). The fixed amount of attenuation is set in step S12 for the following reason. That is, if feedback control is performed when the optical input level is equal to or lower than the threshold, the amount of attenuation reaches the minimum value when the optical input level is low. If an optical signal at a normal level is input in this state, a very high optical signal is output at a subsequent stage. Therefore, there is the possibility that optical parts at the subsequent stage can be destroyed.
If the optical input level reaches the normal level (S13), and the optical channel monitor (OCM) 22 of the output port detects that the optical input level has exceeded the threshold (S14), then the feedback control of the amount of attenuation is started (S15). In the feedback control, the angle of the MEMS mirror 15 is adjusted in the direction y on the basis of the optical signal level detected by the optical channel monitor 22, and the amount of attenuation is controlled (S16). If the optical signal level detected by the optical channel monitor has reached a desired level, the operation is started (S17).
FIGS. 19A and 19B illustrate the operation of controlling a wavelength selection switch. FIGS. 19A and 19B are examples of the case in which the angle of the MEMS mirror 15 is controlled in the array direction of ports.
A wavelength division multiplexed optical signal is output from the ports 2 through 4, but the case in which an optical signal of the channel ch 16 of the port 4 is output to the port 1 is described below for simple explanation.
When the optical input level is higher than the threshold, the angle of mirror corresponding to the channel 16 of the MEMS mirror 15 is feedback-controlled for the light of the wavelength of the channel ch 16 input from the input port 4, and the light attenuated by an appropriate amount of attenuation is input to the port 1. In this case, the light of the channel ch 16 of the ports 2 and 3 is also reflected by the MEMS mirror 15, but the light is not input to the port 1.
When the optical signal is disconnected and the optical input level drops to the threshold or lower, the angle of the mirror corresponding to the channel ch 16 of the MEMS mirror 15 is controlled in the array direction of the ports by the fixed amount of attenuation as illustrated in FIG. 19B. In this case, since the amount of attenuation is large, and the angle of the mirror is also large, there is the possibility that the light of the channel ch 16 of the port 3 enters the output port 1 and the cross talk occurs.
FIG. 20 illustrates an optical coupling image for the output port. In FIG. 20, the white circles indicate the positions of the ports 1 through 4, and the black or gray circles indicate the positions of the reflected light of the MEMS mirror 15.
When the MEMS mirror 15 is controlled in the port direction by the fixed amount of attenuation, the reflected light a′ of the port 3 enters the output port 1 as illustrated in FIG. 20, and the cross talk can occur.
Since the angle of the MEMS mirror 15 is controlled in the array direction of the ports by the fixed amount of attenuation in the above-mentioned control method, there is the possibility that the cross talk occurs during operation.
FIG. 21 illustrates another boot-up sequence. In this example, the angle of the MEMS mirror 15 is controlled in the direction orthogonal to the array direction of the ports.
When the optical input level is equal to or lower than the threshold (S21 in FIG. 21), the control unit 23 controls the MEMS mirror 15 in the direction orthogonal to the array direction of the ports (spectral direction) to assign the fixed amount of attenuation (S22).
If an optical signal at the normal level is input (S23), the optical channel monitor 22 detects the optical signal level of each channel (S24). Then, the feedback control is started depending on the optical signal level detected by the optical channel monitor 22 (S25).
If the feedback control is started, the MEMS mirror 15 is controlled in the direction orthogonal to the array direction of the ports (direction x in FIG. 16) so that the optical signal level of the output port can reach a desired level. If the desired level is reached, the operation is started (S27).
FIGS. 22A and 22B illustrate the controlling operation of the wavelength selection switch. Described below is the case in which the optical signal of the channel ch 16 of the port 4 is output to the port 1.
When the optical input level is higher than the threshold, the angle of the mirror corresponding to the channel ch 16 of the MEMS mirror 15 is feedback-controlled in the direction orthogonal to the array direction of the ports (direction x) as illustrated in FIG. 22A. In this case, the light of the channel ch 16 of the ports 2 and 3 is also reflected by the same mirror of the MEMS mirror 15. However, since the angle of the MEMS mirror 15 is controlled in the direction x, the light output from the port 3 does not enter the port 1.
When the optical input level is equal to or lower than the threshold, the angle of the MEMS mirror 15 is controlled so that a large fixed amount of attenuation can be assigned. In this case, since the MEMS mirror 15 is controlled in the direction orthogonal to the array direction of the ports as illustrated in FIG. 22B, the light of other ports does not enter the output port 1. In this process, no cross talk occurs, but the transmission band characteristic is degraded during the operation.
FIG. 23 illustrates the optical coupling image for the output port. In FIG. 23, the white circles indicate the positions of the respective ports 1 through 4, and the black or gray circles indicate the positions of the reflected light of the MEMS mirror 15.
When the MEMS mirror 15 is controlled in the direction orthogonal to the array direction of the ports (direction x) by the fixed amount of attenuation, the light of other ports does not enter the output port 1 as illustrated in FIG. 23, and therefore no cross talk occurs.
Since the angle of the MEMS mirror 15 is controlled in the direction orthogonal to the array direction by the fixed amount of attenuation in the above-mentioned control method, no cross talk occurs, but the transmission band is degraded in the operation when the optical signal level exceeds the threshold.
FIG. 24 is an explanatory view of the degradation of the transmission band. FIG. 24 indicates the relationship among the frequency, the amount of relative attenuation, and the amount of attenuation of the MEMS mirror 15. The horizontal axis indicates the optical frequency, and the vertical axis indicates the relative attenuation value. 1.934×E+14 refers to 1,934×1014 (193.4 [THz]), which indicates that the band degradation in an ear-shaped form occurs on the right and left of the transmission band together with the amount of attenuation.
When the angle of the MEMS mirror 15 is controlled in the direction orthogonal to the array direction of the ports (spectral direction), the transmission band of the wavelength selection switch 11 is degraded the further as the larger amount of attenuation is assigned.
FIG. 25 is an explanatory view of the degradation of a transmission signal. The horizontal axis indicates an optical frequency and the vertical axis indicates a relative attenuation value [dB]. When the angle of the MEMS mirror 15 is control device in the spectral direction, in a ring network in which a plurality of optical add-drop multiplexers (OADM) are connected in a ring form, a signal is degraded each time the signal passes through each optical add-drop multiplexer. FIG. 25 illustrates the number of spans of the optical add-drop multiplexers through which the optical signal passes and the state of the degradation of the signal.
FIG. 26 is an explanatory view of a Q penalty. A Q penalty refers to a value indicating the degradation of signal quality by band degradation as compared with the case in which no band degradation occurs (Ear=0 dB). FIG. 26 illustrates the relationship between the number of spans indicating the number of optical add-drop multiplexers through which an optical signal passes, Q penalty and the amount of band degradation (size of ear in FIG. 24). As illustrated in FIG. 26, the larger the degradation (ear) or the larger the number of spans, the higher the value of the Q penalty.
FIGS. 27A and 27B illustrate the transmission band characteristic and the mirror control tolerance in the above-mentioned control method.
The method of controlling the angle of the MEMS mirror 15 in the array direction of the ports (direction y) is good in transmission band characteristic both during boot-up and operation (normal operation) as illustrated in FIG. 27A. However, the angle control tolerance of a mirror is low during both boot-up and operation.
The method of controlling the angle of the MEMS mirror 15 in the direction orthogonal to the array direction of the ports (direction x) is high in the angle control tolerance of a mirror is low both during boot-up and operation (normal operation) as illustrated in FIG. 27B. However, the transmission band characteristic is low during both boot-up and operation.
The patent document 1 discloses monitoring the optical power of each channel by a channel monitor monitoring the light reflected by the end surface of the output port of a wavelength selection switch and returned to the input port.
The patent document 2 discloses a wavelength selection switch including a VIPA for holding outputting the wavelength division multiplexed light A and B at the output angle depending on the wavelength, and a focus lens for condensing the light A and B on one point of the micromirror in a micromirror array.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2006-243571
[Patent Document 2] Japanese Laid-open Patent Publication No. 2004-258409
[Patent Document 3] Japanese Laid-open Patent Publication No. 2006-133336